The present invention relates to a method of fabricating image pickup tubes, and particularly to a method of fabricating image pickup tubes with a color separation stripe-shaped filter (will be called a stripe filter hereinafter) formed on the face plate.
The stripe filter falls into the inorganic stripe filter which is formed of striped dichroic thin films and the organic color stripe filter which is formed of striped organic resin films on a glass substrate and colored to have desired hues. Such a stripe filter is arranged on a transparent face plate of the image pickup tube along with a photoelectric conversion element comprised of, for example, a photoconductive film and a transparent conductive film. In this case, it is necessary that the stripe filter be substantially optically contiguous to the surface of the photoelectric conversion element. However, irrespective of the type of the stripe filter, i.e., inorganic or organic, formation of the photoelectric conversion element directly on the stripe filter would impair reliability of the image pickup tube and quality of pictures reproduced, and hence is undesirable. More specifically, in the dichroic stripe filter, filter function portions are raised by about 1 to 3 microns with respect to the remaining portion and if the transparent conductive film for pickup of signals and the photoelectric film are formed directly on the surface of the filter, the photoelectric film will deteriorate along the filter function portions during operation of the image pickup tube and spurious signals will take place. In the case of the organic color filter, it is mainly composed of organic substances including high vapor pressure substances which emit gas when exposed to the vacuum inside the pickup tube, resulting in a malfunction of the tube.
Therefore, the stripe filter of any type needs to be coated with a transparent inorganic material. In conventional dichroic stripe filters, a vapor deposition film of SiO.sub.2 or TiO.sub.2, or a sputtered film of SiO.sub.2 or glass was formed on the filter and polished as desired, and a transparent conductive film was formed on the vapor deposited film.
In the case of the organic color stripe filter, a glass sheet having a substantially finished thickness was adhered to the filter and polished as desired, and a transparent conductive film was formed on the glass sheet. In another method, a thin glass sheet having a substantially finished thickness and provided with a transparent conductive film formed thereon was adhered to the filter.
Conventionally, in the case of an image pickup tube having an approximately 1-inch diameter face plate structure comprised of a face plate, a stripe filter and a photoelectric conversion element, the minimal possible thickness of a glass sheet was preferably about 30 microns or 40 microns at most in order to ensure that the glass sheet can be prepared as having a substantially finished thickness and that the stripe filter can be substantially optically contiguous to the photoelectric conversion element. In order to manufacture flat glass sheets having a thickness of about 30 microns on the industrial scale, for example, a material glass sheet was adhered to a base having a known thickness by means of a temporary binder layer having a known thickness to form a block, the material glass sheet was finished through the steps ranging from rough grinding to final polishing until the block has a desired thickness, and finally the base along with the temporary binder layer was removed from the block to complete a desired thin glass sheet. Typically, while the grinding step uses a diamond tool, a sand abrasive or both in combination, the polishing step uses, for example, fine powder of cerium oxide as an abrasive.
The glass sheet thus manufactured has opposite surfaces of which one is more or less quenched and has compression stress and the other is a mechanically finished surface, and hence has asymmetry in terms of dynamics. And, if opposite surfaces of the glass sheet are finished, a resulting thickness of the glass sheet is so small that mechanical strength of the unitary glass plate is impaired and the glass sheet becomes fragile. Moreover, the above-mentioned method is disadvantageous economically, because the production cost, mainly finishing cost, is raised and the yielding rate is decreased. Accordingly, the impairment of economy is aggravated if one tries to reduce the thickness of the unitary glass sheet to about 30 microns or less. Without reduction to about 30 microns, or 40 microns or less at most, an image of the stripe filter projected by incident light through the pickup lens onto the photoelectric conversion element is blurred, resulting in a reduction in the output color signal amplitude.